African Forest Biodiversity - Earthwatch Institute

Traditional transects can be difficult to place in forests. Timed speciescounts,
which do not depend on a straight-line route, are one solution, but they
have substantial disadvantages too. Some of these can be overcome by simple
modification. For comparative survey and monitoring, however, the biggest
drawback concerns the calculated abundance indices, which are not arithmetically
tractable – therefore, one cannot simply calculate overall indices for, say,
feeding guilds. Timed transects have been tested in several Kenyan forests,
and combine the flexibility of timed species-counts with the additive indices of a
standard, fixed-width transect.
All of these visual/aural methods work best for canopy and mid-level
birds. For undergrowth species, standardised mist netting is appropriate for
survey and monitoring work, although it is labour-intensive and time-consuming.
By confining point counts and timed transects to birds above a certain level
(3m has been used in Kenyan forests), it is possible to differentiate undergrowth
and higher-level birds. As the results of these studies show, bird community
changes can be different at these two levels. This is to be expected,
given that structural change after logging can affect high- and low-level
vegetation in very dissimilar ways.
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